1. Field of the Invention
This invention relates to a solution processing method for forming electrical contacts to organic devices, wherein the electrical contacts formed are suitable for being used in a process for manufacturing organic photovoltaic cells, e.g. fully solution processed organic photovoltaic cells.
2. Description of the Related Technology
Organic photovoltaic cells have reached power conversion efficiencies above 5%, as reported e.g. by J. Peet et al. in “Efficiency enhancement in low-bandgap polymer solar cells by processing with alkane dithiols”, Nature Mater. 6.7 (2007), 497-500 and by J. Y. Kim et al. in “Efficient Tandem Polymer Solar Cells Fabricated by All-Solution Processing”, Science 317.5835 (2007), 222. In order to achieve cheap, large area devices, there is a need for manufacturing methods that are compatible with in-line processing, such as for example methods based on solution processing.
Research efforts related to solution processing of photovoltaic cells have mainly focused on the deposition of the organic active layer, for example based on a P3HT:PCBM solution. Deposition methods that can be used for the fabrication of the active layer of organic photovoltaic cells are for example inkjet printing, flexography, gravure printing, spray coating, doctor blading and slot-die coating.
However, for processing of cheap large area devices it is preferred to provide all the layers of the device by means of methods that are compatible with in-line processing, such as for example methods based on solution processing. The deposition of a cathode typically involves vacuum deposition of a metal or the application of solution processed conductive polymer materials, such as for example doped conjugated polymers, e.g. polyanilines, polypyrroles or PEDOT. These conductive polymer materials are characterized by a low electrical conductivity, e.g. in the order of 0.1 to 10 S cm−1 and poor electrical and/or thermal stability.
In addition, for processing of cheap large area devices it is preferred to use deposition methods that allow local deposition, i.e. deposition methods that allow providing patterned layers. Some solution-based methods that allow local deposition and which have been investigated for forming active layers or electrical contacts include ink-jet printing, micro-contact printing and spray coating.
A particular aspect related to all-solution processing is the need for compatibility of a solution processed layer with the underlying layer or film, i.e. there is a need for avoiding dissolution of underlying layers or films by a solvent of the solution processed layer. Techniques that may be used to solve this issue include the use of orthogonal solvents, cross linkable materials or fast evaporation of solvents.
In e.g. “Plastic-Compatible Low Resistance Printable Gold Nanoparticle Conductors for Flexible Electronics”, Journal of The Electrochemical Society, 150.7 (2003) G412, it has been shown that inks comprising metal nanoparticles can be used for forming solution-based, highly conductive metal patterns. A good electrical conductivity, in the order of 104-106 S cm−1, can be obtained by sintering the ink at relatively low temperatures (for example at temperatures below 150° C.), or by other methods such as laser sintering. These processes can for example be used in the production of printed thin-film transistors, e.g. for forming source and drain electrodes.
Most of the research related to metal nanoparticle inks focuses on the production of bottom-contact devices, where for example ink-jet printing is used to pattern metal lines on a substrate, such as e.g. glass, silicon or plastic foils. As these metal patterns are formed on the substrate, before e.g. an active layer is provided, the process conditions (e.g. sintering process conditions, choice of solvent) are only limited by the properties of the substrate.
In “Organic transistors manufactured using inkjet technology with subfemtoliter accuracy”, Proceedings of the National Academy of Sciences of the United States of America, Vol. 105, No. 13, Apr. 1, 2008, pp 4976-4980, Sekitani et al. report the fabrication of Ag source and drain contacts directly on the surface of an organic semiconductor film by solution processing using a subfemtoliter inkjet printer. The observation that good source and drain contacts can be inkjet printed on top of the organic semiconductor surface is attributed to the small volume of the droplets ejected from the subfemtoliter inkjet head. Because of the small volume (diameter less than 1 micrometer) of the droplets, the organic solvent of the metal ink substantially evaporates before the droplet reaches the semiconductor surface, such that the organic semiconductor films are not damaged by the solvents and there is no significant spreading of the nanoparticle droplets on the surface. Furthermore, the small size and large surface area of the droplets on the surface reduces the temperature required to remove the dispersing agent and fuse the nanoparticles into a metallic line with good electrical conductivity. It is reported that a temperature of 130° C. is sufficient to obtain a resistivity of 25 micro-Ohm cm. However, the technique reported by Sekitani et al. is not easily scalable to the processing of organic photovoltaic cells, where there is a need for a fast deposition technique able to cover large area contacts, e.g. contacts with an area of several square centimeters or larger.
In “preparation and characterization of nano-scale ZnO as a buffer layer for inkjet printing of silver cathode in polymer solar cells”, Solar Energy Materials and Solar Cells 92 (2008), pp 564-570, S. H. Eom et al. report organic photovoltaic cells with a silver cathode formed by inkjet printing, wherein the cathode is formed after providing the organic semiconductor layer. Because of the hydrophilic character of both the silver ink and the surface of the organic semiconductor layer on which the silver cathode is to be formed, a hydrophilic ZnO layer is used as a buffer layer between the organic semiconductor layer and the silver cathode. However, the performance of the organic photovoltaic cells thus obtained is rather limited. It is shown that the cell efficiency increases with increasing annealing temperature of the ZnO layer. For annealing at 150° C., cell efficiencies of 0.2% are reported. This temperature of 150° C. can be considered as an upper temperature limit when using PEN (PolyEthylene Naphthalate) as a substrate. In case of a PET (PolyEthylene Terephthalate) substrate, the upper temperature limit is about 110° C.